Vectors (std::vector)

is the container you'll reach for most in C++: a resizable array that grows on demand, knows its own size, and plugs into the entire standard-algorithm library. This lesson takes you from creating and looping over a vector to sorting, summing, counting, and even building a 2D grid.

Learn Vectors (std::vector) in our free C++ course — a beginner-friendly interactive lesson with worked examples, a practice exercise and a quick recall.

Part of the free C++ course at LearnCodingFast — hands-on lessons with examples you run in your browser, plus practice exercises and a quick quiz.

A std::vector is an expandable shelf . You start with a few slots; when you run out, the shelf quietly buys a bigger unit, moves all your books across, and carries on — you never have to measure and order the shelf yourself. A raw array is a fixed bookcase : pick a size once, and if you guess too small you're stuck. The catch with the expandable shelf is that when it moves to a bigger unit, every book changes location — so a sticky note pointing at "third shelf, second slot" (a reference or iterator) may no longer point where you expect after a big growth.

1. Creating & Accessing

Create a vector with brace initialization and read elements by index from 0 . .front() and .back() grab the ends, .size() reports how many elements there are, and .at(i) is the bounds-checked alternative to [i] .

2. Growing & Shrinking

Start with an empty vector and append with push_back ; the vector handles all the memory management for you. pop_back removes the last element, and .empty() tells you whether there's anything left. This dynamic resizing is exactly what raw arrays can't do.

3. Three Ways to Loop

An index loop gives you the position (useful when you need i ). A range-based loop with const auto& reads each element efficiently without copying. A range-based loop with auto& lets you modify elements in place. Reach for the simplest form that does the job.

4. Vectors + Algorithms

The real power of vectors is how well they pair with the standard algorithms. sort orders them, accumulate (from ) sums them, and count_if with a lambda counts matches. Learning to combine a container with these algorithms is the heart of effective C++.

5. 2D Grids: Vector of Vectors

A models a grid or matrix. The constructor builds a rows × cols grid filled with zeros. Index it with two subscripts, grid[r][c] , and loop with nested range-based loops.

Your turn. Build a small list and report on it:

These lines build a vector, sort it, and print the largest value. Reorder so the program prints 9 .

Why: must precede the use of sort . Inside main , you build the vector first, then sort it ascending so the largest value lands at the end — which v.back() reads as 9 . Return and brace close the function.

Predict the output before revealing each answer.

{'vector v = ; v.push_back(4); cout << v.size();'}

4 — three initial elements plus one push_back makes four.

{'vector v = ; sort(v.begin(), v.end()); cout << v[0];'}

1 — after an ascending sort the smallest element is at index 0.

{'vector v = ; for (auto& n : v) n += 1; cout << v[0] << v[1];'}

35 — auto& modifies each element in place, so the vector becomes {' '}.

Compute the high, low, average, and pass count for a vector of grades using the algorithm library. Match the expected output in the comments.

Practice quiz

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